Motorcycle frame assembly

ABSTRACT

A motorcycle frame assembly arranged to have a desirable amount of rigidity in both transverse and vertical directions. The frame assembly desirably includes a head pipe configured to support a front wheel and associated suspension assembly. A main tube and down tube extend in a rearward direction from the head pipe. Desirably, the main tube is elliptical in cross-section, with the longer dimension being arranged generally horizontally and the shorter dimension being arranged generally vertically. Preferably, the main tube is arcuate along its length and defines a convex outer surface portion opposing a concave outer surface portion. Preferably, the main tube is arranged such that the convex outer surface portion faces the down tube. Desirably, the main tube also extends from the head pipe such that the main tube is lower than perpendicular with respect to the head pipe.

PRIORITY INFORMATION

[0001] This application is related to, and claims priority from,Japanese Patent Application No. 2001-173832, filed on Jun. 8, 2001, theentirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to motorcycle frameassemblies and, more particularly, to an improved motorcycle frameassembly that achieves a desirable balance between rigidity in a lateraldirection and rigidity in a vertical direction.

[0004] 2. Description of the Related Art

[0005] As is well known, motorcycles generally include a frame assemblythat is made up of a plurality of individual tubes welded together. Thetypical frame construction includes a head pipe at a front end of theframe, which desirably supports the front wheel via a steerable frontfork assembly. One or more main tubes typically extend generallyrearwardly from the head pipe. The main tubes may bend downward over arear portion of an engine that is positioned generally below the maintubes. In addition, one or more down tubes are typically affixed attheir forward ends to the head pipe at a location lower than the maintubes. The down tubes extend downwardly and rearwardly from the headpipe. The rear ends of the main tube and the down tubes generally arejoined, frequently by a pair of seat rails and bracket assemblies. Oneof the bracket assemblies typically provides support for the rear wheelsuspension swing arm assembly. Although the arrangement of theindividual frame members may vary, the above-described construction isgenerally the type utilized for motorcycles in general and off-roadmotorcycles in particular.

[0006] In off-road motorcycles, it is often desirable for the motorcycleframe to possess differing levels of rigidity in the lateral directionand the vertical direction. This is due, at least in part, to anoff-road motorcycle being subjected to forces occurring from bothcornering and jumping.

[0007] In the case of lateral loads, rigidity is desired to improvestability during cornering at high speeds or over rough terrain. In thecase of the vertical loads, the motorcycle may land from considerableheights, which causes large shock loads to be applied to the frame. Ifthe frame is too rigid, the shock loads are largely transferred to therider. If the frame is not rigid enough, the frame may elasticallydeform or break.

[0008] U.S. Pat. No. 5,845,728, issued to Hiroyuki Itoh et al. on Dec.8, 1998, describes several embodiments intended to increase thetransverse rigidity of a motorcycle frame while not overly increasingthe vertical rigidity of the motorcycle frame. The disclosed embodimentsfeature main tubes that feature flat upper and lower faces. Thus, insome embodiments, the main tube is rectangular with a larger lateraldimension than a vertical dimension. In the arrangement shown in FIG. 12of the '728 patent, the main pipe is disclosed as having an ellipticalcross-section with flat upper and lower surfaces that facilitateattaching components of the motorcycle to the frame and rounded sidewalls that supposedly enable a characteristic design. It is clear fromthis description and the cross-section shown in FIG. 13 that the mainpipe is oval and not elliptical in shape.

SUMMARY OF THE INVENTION

[0009] The oval shape and the rectangular shape of the main pipe in the'728 patent increase the transverse rigidity of the frame assembly whilenot overly increasing the vertical rigidity of the frame assembly. Thedisclosed shapes, however, each feature a stress-rising construction. Inthe case of the rectangle, the corner at which two perpendicular facesmeet defines a distinct stress-riser. In the case of the oval, therather abrupt change in curvature defined between the flat upper andlower surfaces and the curved side faces also defines a stress-riser. Inorder to adequately compensate for the stress-risers in an off-roadmotorcycle frame, the main pipe diameter would need to be greatlyincreased. The increase in diameter, however, approximates a circulardesign, which suffers from being too rigid vertically when adequatelyrigid laterally.

[0010] Accordingly, preferred embodiments of the present motorcycleframe assembly include a main tube that, in section, is non-circular butcontinuously curved with a lateral dimension that is larger than thevertical dimension. Such an arrangement advantageously provides lateralrigidity while permitting at least a small amount of compliance in thevertical direction. In addition, desirably the main tube curvesdownwardly from the front end toward the center of the main tube andcurves upwardly from the center toward the rearward end of the maintube. With such an arrangement, the ability of the main tube toexperience elastic deformation in response to vertical loads isincreased in comparison to main tube that is linear. As a result, themotorcycle frame exhibits exceptional lateral rigidity to improvehandling, while providing an advantageous amount of vertical complianceto improve the comfort of the rider.

[0011] An aspect of the present invention involves an off-roadmotorcycle comprising a frame assembly. The frame assembly includes ahead pipe, a main tube and a down tube. The main tube and the down tubeextend in a generally rearward direction from the head pipe and the maintube is positioned above the down tube. A front suspension assembly isrotatably supported by the head pipe and rotatably supports a frontwheel. A rear suspension assembly is pivotably supported by the frameassembly and rotatably supports a rear wheel. An internal combustionengine is supported by the frame assembly and is configured to drive therear wheel. The main tube defines a first cross-sectional dimension in afirst direction substantially aligned with a vertical, center plane ofthe motorcycle and a second cross-sectional dimension in a seconddirection substantially normal to the first direction. The seconddimension is greater than the first dimension. The main tube isnonlinear along its length and defines a center axis extending from aforward end of the main tube to a rearward end of the main tube. Acenter portion of the center axis is spaced below a straight lineextending between the forward end and the rearward end of the axis.

[0012] Another aspect of the present invention involves a motorcycleframe assembly comprising a head pipe, a main tube and a down tube. Themain tube and the down tube extend in a generally rearward directionfrom the head pipe. The main tube is positioned above the down tube. Themain tube defines a first cross-sectional dimension in a longitudinalvertical plane of the motorcycle and a second cross-sectional dimensionin a second direction substantially normal to the first direction. Thesecond dimension is greater than the first dimension and the main tubehas a noncircular continuously curved cross-section along at least aportion of the main tube.

[0013] A further aspect of the present invention involves a motorcycleframe assembly comprising a head pipe, a main tube, a down tube and aseat rail. The main tube and the down tube extend in a generallyrearward direction from the head pipe. The main tube is positioned abovethe down tube and the seat rail extends between rearward ends of themain tube and down tube. The main tube, down tube and seat rail arearranged in a generally triangular configuration and define a generallytriangular internal space therebetween. The main tube defines a firstcross-sectional dimension in a first direction. The first direction issubstantially aligned with a vertical, center plane of the motorcycle. Asecond cross-sectional dimension is defined by the main tube in a seconddirection substantially normal to the first direction. The seconddimension is greater than the first dimension. The main tube is arcuatealong its length such that the main tube defines a convex outer surfaceportion opposing a concave outer surface portion. The main tube isarranged such that the convex outer surface generally faces the internalspace.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] These and other features, aspects and advantages of the presentinvention are described with reference to the drawings of a preferredembodiment, which embodiment is intended to illustrate and not to limitthe present invention. The drawings comprise six figures.

[0015]FIG. 1 is a side elevational view of an off-road motorcycleincorporating a preferred frame assembly.

[0016]FIG. 2 is a side elevational view of the frame assembly of themotorcycle of FIG. 1. The frame includes a head pipe, a main tube, adown tube assembly, and a pair of seat rails.

[0017]FIG. 3a is a plan view of the down tube assembly, which includesan upper down tube connected to a pair of lower down tubes, or enginecradle tubes.

[0018]FIG. 3b is a top plan view of a junction between a rearward end ofthe main tube and the pair of seat rails. A lug connects the rearwardend of the main tube to upper ends of the seat rails.

[0019]FIG. 3c is a top plan view of a junction between the lower downtubes and the seat rails of the frame assembly.

[0020]FIG. 4 is a cross-sectional view taken along the view line 4-4 ofFIG. 3 showing a rearward portion of the main tube and the lugconnecting the main tube to the seat rails. The lug includes an upperlug portion connected to a lower lug portion.

[0021]FIG. 5 is a top plan view of a central portion of the main tubeillustrating a reinforcement bracket connected to an upper surface ofthe main tube.

[0022]FIG. 6 is a side plan view of the head pipe and forward ends ofthe main tube and down tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] With reference to FIG. 1, an off-road motorcycle incorporating apreferred frame assembly is identified generally by the referencenumeral 10. The motorcycle 10 includes a frame assembly, indicatedgenerally by the reference numeral 12. The frame assembly preferably isof the semi-double cradle type. Other arrangements also can be used;however, the semi-double cradle type increases the protection of theengine, which is very desirable in off-road motorcycles. The frameassembly 12 is formed primarily from tubular members, which may haveround, square or other suitable cross-sectional configurations. Theconstruction of the frame 12 is described in greater detail below withreference to FIGS. 2-6.

[0024] A front fork assembly 14 is journaled for rotation with respectto the frame assembly 12. The front fork assembly 14 preferably is ofthe telescopic type. A handlebar assembly 16 is carried at an upper endof the front fork 14 for steering of a front wheel 18, which isrotatably journaled at a lower end of the fork 14, as is well known inthe art. The front fork assembly 14 also supports a front fender 20 at aposition above the front wheel 18. The front fender 20 typically isarranged to deflect dirt, mud or other debris that may be thrown upwardby the front wheel 18.

[0025] A swing arm assembly 22 is pivotally supported by a lower,rearward portion of the frame assembly 12 at a pivot axis 24. At itsrearward end, the swing arm 22 supports a rear wheel 26 of themotorcycle 10. A rear shock absorber 28 is operably connected betweenthe frame assembly 12 and the rear swing arm 22. Preferably, the rearshock absorber 28 is connected to the swing arm 22 through a link 30, asis known in the art. Thus, the rear shock absorber 28 provides someresistance to articulating movement of the swing arm 22 and the rearwheel 26 relative to the frame assembly 12. The link 30 may be utilizedto alter a leverage ratio between the rear wheel 26 and the rear shockabsorber 28. Alternatively, the rear shock absorber 28 may be connecteddirectly to the swing arm 22.

[0026] The frame 12 also supports a rear fender 32 at a position abovethe rear wheel 26. The rear fender 32 is arranged to deflect mud, dirtor other objects which may be thrown in an upward direction by the rearwheel 26.

[0027] A fuel tank 34 is supported by the frame 12 in a positiongenerally behind and below the handlebar assembly 16. Desirably, thefuel tank 34 surrounds at least a portion of the frame 12 to lower thecenter of gravity of the motorcycle 10.

[0028] A straddle-type seat assembly 36 generally is positioned betweenthe fuel tank 34 and the rear fender 32. Desirably, the seat assembly 36has a relatively flat upper surface and is thicker in a center portionin comparison with either the front or rear end portions. Such anarrangement permits a rider of the motorcycle 10 to move relativelyfreely between various positions with respect to the seat assembly 36.

[0029] An internal combustion engine, indicated generally by thereference numeral 40, is supported by the frame assembly 12. Preferably,the engine 40 is positioned below the fuel tank 34 and seat assembly 36to provide the motorcycle 10 with a relatively low center of gravity,which improves handling.

[0030] The illustrated engine 40 operates on a 2-stroke, crankcasecompression principle and is of a single cylinder arrangement. However,other suitable engine arrangements may also be used, such as amulti-cylinder arrangement, or engines operating on other operatingprinciples, such as a 4-stroke engine, for example. In addition, theengine can be liquid cooled in some arrangements.

[0031] A chain and sprocket drive assembly 42 extends between the engine40 and the rear wheel 26 and is configured to transfer torque producedby the engine 40 to the rear wheel 26 to propel the motorcycle 10 in anysuitable manner. Although a chain and sprocket type drive assembly 42 isillustrated, other suitable types of drive arrangements may also beused, such as a belt drive or a shaft drive arrangement, for example.

[0032] With reference to FIG. 2, the frame 12 is illustrated with theother components of the motorcycle 10 removed. The frame assembly 12 isconstructed primarily of a main frame portion 44 and a subframe portion46. Desirably, the main frame portion 44 supports the engine 40, thefront fork assembly (which also includes front suspension components)14, the rear shock absorber 28 and the rear swing arm 22. The subframe46 is connected to the main frame 44 and generally supports the seat 36and rear fender 32. As is known in the art, the individual tubes thatcomprise the frame 12 are typically welded together. However, othersuitable construction methods may also be used.

[0033] The main frame 44 desirably includes a head pipe 48, which islocated generally toward a front, upper portion of the main frame 44.The head pipe 48 rotatably supports the front fork assembly 14 and maybe inclined with respect to vertical to support the front fork assembly14 at a desired rake angle with respect to vertical. In other words, acentral axis R extending through the head pipe 48 extends at a suitableangle relative to a vertical axis V. The angle between these axes R, Vhelps to determine the rake angle of the front fork assembly 14.

[0034] A main tube 50 extends rearwardly from an upper portion of thehead pipe 48. In addition, desirably a main body portion of the maintube 50 extends in a generally downward direction relative to ahorizontal axis. The main tube 50 will be described in greater detailbelow.

[0035] A down tube assembly extends generally downwardly and rearwardlyfrom a lower portion of the head pipe 48 and includes an upper down tubeportion 52 and a lower down tube portion. The upper down tube portion 52preferably is formed of a square or rectangular member that is welded orotherwise suitably secured to the head pipe 48. In the illustratedarrangement, the upper down tube portion 52 is formed of a rectangularpipe having a larger transverse dimension than a vertical dimension suchthat the larger two faces of the pipe extend in a generally transversedirection. The lower down tube portion preferably is comprised of a pairof lower down tube members 54, 56. The lower down tube members 54, 56preferably are circular in cross-section although other shapes can beused, such as square, rectangular, elliptical or oval, for instance.

[0036] With additional reference to FIG. 3a, desirably, the lower downtube members 54, 56 extend in a generally lateral direction with respectto a central plane Cp of the motorcycle 10 from a lower end of the upperdown tube 52. The square or rectangular cross-section of the upper downtube portion 52 facilitates coupling of the lower down tube members 54,56 to the upper down tube portion 52. The lower down tube members 54, 56extend rearward in a spaced apart orientation along a directionsubstantially aligned with the upper down tube portion 52. At a lowerend of the frame 12, the lower down tubes members 54, 56 curve andextend rearwardly to support a lower portion of the engine 40. Thus, thelower down tube members 54, 56 extend from a forward side of the engineto a lower side of the engine 40 in a cradle-type frame arrangement.

[0037] A pair of seat pillar tubes 58, 60 are connected to a rearwardend of the main tube 50 by a lug 62. The lug 62 preferably is formed byoverlapping and welding together a plate-like component on both theupper side and the lower side, as will be described. These plate-likecomponents sandwich portions of each of the main tube 50 and the twoseat pillar tubes 58, 60. Preferably, the plate-like components areformed by a press-forming operation or forging.

[0038] With additional reference to FIG. 3b, the seat pillar tubes 58,60 extend in opposing lateral directions from the main tube 50. Lowerends of the seat pillar tubes 58, 60 are connected to respective upperends of a pair of swing arm mount brackets 64, 66, which are spacedapart from one another in a lateral direction approximately the width ofthe engine 40.

[0039] With continued reference to FIG. 3c, a cross tube 68 extendsbetween the pair of brackets 64, 66 to secure the brackets 64, 66 in aspaced apart relationship. A pair of tube members 70, 72 extend fromrespective lower ends of the brackets 64, 66 in a forward direction andconnect to the lower, horizontal portions of the lower down tube members54, 56. In addition, rearward ends of the lower down tubes 54, 56terminate at, and are connected to, the cross tube 68.

[0040] Desirably, the bracket 64 includes an inner bracket portion 64 aand an outer bracket portion 64 b connected to one another and receivinga lower end of the seat rail 58 and a rearward end of the support tube70 therebetween. Similarly, the bracket 66 includes an inner bracketportion 66 a and an outer bracket portion 66 b connected together andsecuring the seat rail 60 and support tube 72 therebetween. Preferably,the rear arm brackets 64, 66 are formed by overlapping and welding apair of plate-like components that have been press-formed or forged.

[0041] The brackets 64, 66 define an aperture 74 for receiving a swingarm support shaft (not shown), which pivotally supports the swing arm 22in any suitable manner. In addition, a pair of foot peg mounts 76 arelocated on the brackets 64, 66. The foot peg mounts 76 are configured tosupport a pair of foot pegs 77 (see FIG. 1) on which a rider of themotorcycle 10 may place his or her feet.

[0042] With reference to FIG. 2, a shock mount bracket 78 preferably isconnected to the main frame 44 at a position near the junction betweenthe main tube 50 and the seat rails 58, 60. More preferably, the shockmount bracket 78 is connected to the lug 62. Even more preferably, theshock mount bracket 78 is connected to an upper portion of a rear end ofthe lug 62 while an upper engine mount is secured to a lower portion ofthe lug 62. The shock mount bracket 78 extends generally in a rearwarddirection from the main frame 44 and defines a boss 80 for mounting anupper end of the rear shock absorber 28. Any suitable shock mountconstruction can be used.

[0043] Desirably, an upper portion, or seat rail 82, of the sub frame 46is connected to the shock mount bracket 78 by a fastener, such as bolt84. A stay portion 86 of the subframe 46 is connected to the brackets64, 66 by a fastener, such as bolt 88, and extends upwardly to connectto a rearward portion of the seat rail 82. Thus, the stay 86 supportsthe rearward portion of the seat rail 82.

[0044] With reference still to FIG. 2, a gusset arrangement preferablyreinforces the connection between the main tube 50, the down tube 52 andthe head pipe 48. For example, an upper gusset 90 extends from an upperend of the head pipe 48 and is connected to an upper surface of the maintube 50. Similarly, a lower gusset 92 extends from a lower end of thehead pipe 48 and is connected to a lower surface of the down tube 52. Anintermediate gusset 94 extends rearward from the head pipe 48 at aposition between the main tube 50 and the down tube 52 and is connectedto both the main tube 50 and the down tube 52. Desirably, theintermediate gusset 94 is substantially larger than either of the upperor lower gussets 90, 92.

[0045] Preferably, the frame 12 includes a plurality of engine mountsconfigured to secure the engine 40 to the frame 12. For example, a pairof lower engine mounts 96 are fixed to the lower down tube 54. Althoughnot shown, an additional pair of engine mounts 96 desirably are affixedto the lower down tube 56 in a similar manner. In addition, an upperengine mount 98 is desirably fixed to the lug 62, which connects themain tube 50 to the seat rails 58, 60. The engine 40 may be mounteddirectly to the brackets 96, 98 or an additional connection member mayextend between one or more of the mount brackets 96, 98 and the engine40.

[0046] With reference to FIG. 4, the main tube 50 and the lug 62 areshown in section. As illustrated, the main tube 50 is desirablyelliptical in cross-section with the major axis being arranged generallyhorizontally. As in known, an ellipse is mathematically a closed curvethat is generated by a point moving in such a way that the sums of itsdistances from two fixed points is a constant. In one preferredconstruction, the cross-section of the main tube 50 is a non-circularcontinuously curved shape. For instance, the cross-section does not havea planar side that meets with a curved side. By reducing or completelyremoving sudden changes in curvature (such as at a corner between twoperpendicular surfaces or an intersection between a straight line and aradius), the structural integrity of the main pipe 50 is lesscompromised by stress-risers that can be found at the sudden changes incurvature.

[0047] In the illustrated arrangement, the main tube 50 defines a firstdiameter D1 in a horizontal, or lateral direction, and a second diameterD2 in a vertical direction or, a direction generally parallel with acentral vertical plane Cp of the motorcycle 10. Desirably, the diameterD1 is larger than the diameter D2. With such an arrangement, lateralrigidity of the main tube 50, and thus the frame 12, is increased whilealso increasing vertical compliance of the main tube 50 and, thus, theframe 12. As a result, the motorcycle 10 utilizing a preferred frame 12exhibits exceptional handling due to the increased lateral rigidity andis also comfortable for the rider due to the increased verticalcompliance.

[0048] As also illustrated in FIG. 4, desirably, the lug 62 includes anupper lug portion 62 a and a lower lug portion 62 b connected to oneanother through any suitable means, such as welding, for example. Theupper and lower portions 62 a, 62 b of the lug 62 secure a rearward endof the main tube 50 and upper ends of the seat rails 58, 60therebetween.

[0049] With additional reference to FIG. 3b, the upper portion 62 a ofthe lug 62 desirably includes a cut-out portion, or channel 100, along aportion of the upper surface of the main tube 50. The channel 100prevents the lug 62 from inhibiting bending of the main tube 50 along ahorizontal axis and thereby substantially avoids negatively impactingthe vertical compliance of the main tube 50. Preferably, as illustrated,the channel 100 extends beyond the rearward end of the main tube 50.

[0050] With reference to FIG. 2, desirably, the main tube 50 is arcuatealong its length. That is, the main tube 50 is preferably non-linearand, more preferably, has a constant curvature such that a centerportion of the tube 50 is disposed lower than the end portions (i.e.,toward the down tube 52) relative to a straight line extending betweenthe ends of the main tube 50. The curvature of the main tube 50 resultsin one-half of the main tube 50 defining a convex outer surface and theopposing one-half defining a concave outer surface. Desirably, theconvex outer surface of the main tube 50 faces a generally triangularinterior space defined by the main frame 44. Advantageously, thecurvature of the main tube 50 permits greater elastic deformation, orflexing, of the tube 50 when a load is applied to one or both ends.Accordingly, the fatigue life of the main tube 50 is increased incomparison with frames utilizing a straight, or linear, main tube.

[0051] During operation of the motorcycle, the loads applied to theframe, through the wheels 18, 24 and suspension members 14, 28, areabsorbed primarily by the main tube 50. Accordingly, the curved maintube 50 increases the fatigue life of the entire frame assembly 12.Further, in combination with the elliptical cross-section, the curvedmain tube 50 enhances the vertical compliance of the frame assembly 12.Accordingly, less force is transmitted to the rider when the motorcycle10 is subjected to a large impact in comparison with prior frame designsutilizing linear main tubes having a circular cross-section.

[0052] The curved main tube 50 also permits ergonomic benefits to beachieved. For example, because of the curvature of the main tube 50, thefuel tank 34 may be mounted lower thereby lowering the overall center ofgravity of the motorcycle 10. Lowering the fuel tank 34 even a smalldistance produces a large improvement to the handling characteristics ofthe motorcycle 10, due to the weight of the fuel and its tendency toshift in response to movements of the motorcycle 10. In addition,because the fuel tank is lowered, the portion of the seat 36 above thefuel tank 34 may be thicker without an increase in height. Accordingly,more padding can be provided in this location to increase comfort forthe rider of the motorcycle 10.

[0053] With additional reference to FIG. 5, desirably a reinforcementbracket 102 is attached to a central portion of the main tube 50.Preferably, the bracket 102 is mounted to an upper surface of the maintube 50 and extends around a portion of the circumference of the tube50. In the illustrated arrangement, the bracket 102 extends slightlyless than halfway around the circumference of the tube 50. In onepreferred arrangement, the bracket is mounted at the location in whichthe main tube 50 is bowed downward such that the main tube 50 can bereinforced in the central portion of this bend region.

[0054] The bracket 102 includes a pair of cut-out portions 104,substantially aligned with an axis of the main tube 50. In theillustrated arrangement, the cut-out portions 104 open at forward andrearward ends of the bracket 102, respectively, and extend to a roundedend portion. However, the bracket 102 may take on other suitablearrangements.

[0055] Advantageously, the bracket 102 is configured to reinforce thestrength of the main tube 50 in response to bending or torsional loads.In addition, the cut-out portions 104 permit the main tube 50 to flex,despite the presence of the bracket 102. Thus, the bracket 102 allowssome flexion while not allowing too much flexion such that the main tube50 can be elastically deformed or broken. Accordingly, the size andshape of the cut-out portions 104 may be altered, along with thecross-sectional shape and curvature of the main tube 50, to achieve adesirable balance between lateral rigidity, vertical compliance andoverall strength of the frame assembly 12. For instance, the cut-outportions can be V-shaped, circular, or irregular in configuration.

[0056] With reference to FIG. 6, an axis L1 of the main tube 50intersects with an axis C of the head pipe 48 at a point P. Desirably,the main tube 50 extends in a rearward direction from the head pipe 48such that the axis L1 is disposed below (i.e., toward the down tube 52)an imaginary line L2 extending from the point P and being perpendicularto the axis C of the head pipe 48. In the illustrated arrangement, themain tube 50 actually is bowed such that it is concave upward (i.e., theends are higher than the middle—similar to a smile). This provides thatthe main tube 50 is connected to the head pipe 48 with an upwardlyextending end of the main tube 50. Advantageously, such an arrangementenhances the vertical compliance of the frame assembly 12 by encouragingthe main tube 50 to flex in response to a load being applied at theforward end of the main tube 50, i.e., through the head pipe 48.

[0057] Although the present invention has been described in the contextof a preferred embodiment, it is not intended to limit the invention tothe provided example. Modifications to the above-described frameassembly apparent to one of skill in the art are considered to be partof the present invention. Accordingly, the invention should be definedsolely by the appended claims.

What is claimed is:
 1. An off-road motorcycle comprising a frameassembly, the frame assembly including a head pipe, a main tube and adown tube, the main tube and the down tube extending in a generallyrearward direction from the head pipe, the main tube being positionedabove the down tube, a front suspension assembly rotatably supported bythe head pipe and rotatably supporting a front wheel, a rear suspensionassembly pivotably supported by the frame assembly and rotatablysupporting a rear wheel, an internal combustion engine supported by saidframe assembly and being configured to drive the rear wheel, the maintube defining a first cross-sectional dimension in a first directionsubstantially aligned with a vertical center plane of the motorcycle anda second cross-sectional dimension in a second direction substantiallynormal to the first direction, the second dimension being greater thanthe first dimension, the main tube being non-linear along its length anddefining a center line extending through the main tube from a forwardend of the main tube to a rearward end of the main tube, a centerportion of the center line being spaced below a straight line extendingbetween the forward end and the rearward end of the axis.
 2. Theoff-road motorcycle of claim 1, wherein the head pipe defines a centeraxis, the center line of the main tube intersecting the center axis ofthe head pipe at an intersection point, the main tube extending from thehead pipe such that the center line of the main tube is inclined in adownward direction with respect to a line passing through theintersection point and being normal to the axis of the head pipe.
 3. Theoff-road motorcycle of claim 1, additionally comprising a reinforcementbracket attached to a center portion of the main tube and extendingalong a portion of the length of the main tube.
 4. The off-roadmotorcycle of claim 3, wherein the reinforcement bracket is positionedon an upper surface of the main tube.
 5. The off-road motorcycle ofclaim 3, wherein the reinforcement bracket includes a cut-out portionextending from an end of the reinforcement bracket and being generallyaligned with the center axis of the main tube.
 6. The off-roadmotorcycle of claim 1, additionally comprising a seat pillar extendingbetween the down tube and the main tube, a lug connecting a rearward endof the main tube to an upper end of the seat pillar, the lug having anupper lug portion secured to a lower lug portion and securing therearward end and the upper end therebetween.
 7. The off-road motorcycleof claim 6, the lug having a cut-out portion extending from a forwardend of the lug and generally aligned with the center axis of the maintube.
 8. The off-road motorcycle of claim 7, wherein the cut-out portionextends beyond a rearward end of the main tube.
 9. A motorcycle frameassembly comprising a head pipe, a main tube and a down tube, the maintube and the down tube extending in a generally rearward direction fromthe head pipe, the main tube being positioned above the down tube, themain tube defining a first cross-sectional dimension in a longitudinalvertical plane of the motorcycle and a second cross-sectional dimensionin a second direction substantially normal to the first direction, thesecond dimension being greater than the first dimension, and the maintube having a noncircular continuously curved cross-section along atleast a portion of the main tube.
 10. The motorcycle frame of claim 9,wherein the head pipe comprises a center axis, the main tube comprises acenter line that intersects the center axis of the head pipe at anintersection point, the main tube extending from the head pipe such thatthe center line of the main tube slopes in a downward and rearwarddirection with respect to an imaginary transverse plane passing throughthe intersection point and being normal to the axis of the head pipe.11. The motorcycle frame of claim 9 additionally comprising areinforcement bracket attached to a center portion of the main tube andextending along a portion of the length of the main tube.
 12. Themotorcycle frame of claim 9, wherein the portion having the noncircularcontinously curved cross-section is bowed downward relative to astraight line extending orthogonal to a center axis of the head pipe.13. The motorcycle frame of claim 12 further comprising a reinforcementbracket attached to the portion having the noncircular continuouslycurved cross-section.
 14. The motorcycle frame of claim 12, wherein aforward end of the main tube is connected to the head pipe and theforward end of the main tube extends to the head pipe in an upwarddirection such that an angle is defined between a line extendingorthogonally away from the head pipe axis and a transverse centerplaneextending though the main tube.